1. Field of the Invention
The present invention relates to an arrangement for determining visibility, preferably oblique visibility on airfields.
On airfields it is extremely important to be able continuously to supervise the visibility and to have accurate information as to visibility at the particular moment.
2. The Prior Art
Many different devices have been proposed to solve the problem of determining the visibility prevailing. One such device comprises a plurality of light sources arranged at different and known distances. Visibility is thus determined by the distance to the furthest light source which is visible at the moment in question. This method is thus dependent on the eyesight of the person performing the observation. Another such device comprises a light source which illuminates a reflector arranged at a relatively short distance from the light source, and a receiver which measures the intensity of the light reflected from said reflector. If visibility deteriorates, the intensity diminishes. One disadvantage with this method is that the distance between the emitter and the reflector must be short since this distance is the least which can be measured at reduced visibility. Another disadvantage is that a local disturbance, for example a bank of mist across the short measuring distance, is recorded as a general reduction in visibility.
One possibility of avoiding these disadvantages is disclosed in U.S. Pat. No. 3,694,936. In this a light emitter emits a series of light pulses towards reflectors placed at different distances. Light pulses reflected from the reflectors are detected by a receiver which, by knowing the time of emission of the pulses, is able to identify the reflector or reflectors which have caused the reflected light pulses.
In connection with landing on an airfield it is desirable, from the point of view of security, to have a knowledge of the height and distance to the point in the glide path where the pilot can see the end of the landing runway (the oblique visibility). In concrete terms this involves, among other things, information as to the visibility along a line in the direction of approach from a point in immediate connection to the end of the landing runway (the point of set-down) and to a point 3-5 km outside this point in the extension of the landing runway.
One possibility of determining this oblique visibility would be to utilize the principle according to U.S. Pat. No. 3,694,936, and in that case the reflectors would have to be placed in towers of different heights. Such a method is however not realistic from the point of view of security and expense since a long range of visibility, for example 1,000 m with a gliding angle of 3.degree., would require a tower of a height of 1000 .times. sin 3.degree. = 52 m.